An organic electroluminescent (EL) element is a self-luminescent element utilizing such a principle that a fluorescent substance emits light with recombination energy of holes injected from an anode and electrons injected from a cathode on application of an electric field. An organic EL element containing an organic material as a constitutional material has been actively investigated since the report by C. W. Tang, et al., Eastman Kodak Corporation, for a low voltage driving organic EL element with a stacked device (see Non-patent Document 1).
Tang, et al. discloses an organic EL element having a laminated structure using tris(8-quinolinolato)aluminum in a light emitting layer and triphenyldiamine in a hole transporting layer. The advantages of the use of the laminated structure in the organic EL element include the following: (i) the injection efficiency of holes into the light emitting layer is enhanced; (ii) electrons injected from the cathode to the light emitting layer are blocked by the hole transporting (injection) layer, and thereby the formation efficiency of excitons formed through recombination in the light emitting layer is enhanced; and (iii) the confinement of the excitons formed in the light emitting layer inside the light emitting layer is facilitated. For enhancing the recombination efficiency of holes and electrons thus injected in the organic EL element having the laminated structure, researches have been made for the structure and the formation method of the element, and the materials of the layers.
In general, the storage or operation of an organic EL element under a high temperature environment may cause adverse affects, such as change of the light emission color, decrease of the light emission efficiency, increase of the operation voltage, and reduction of the lifetime of the element.
For preventing the adverse affects, there have been proposals of aromatic amine derivatives having a carbazole skeleton, a fluorene skeleton, a dibenzofuran skeleton or a dibenzothiophene skeleton, as an organic EL element material (see Patent Documents 1 to 3).